DNSSEC RSA: RSA/SHA-1 SIGs and RSA KEYs in the Domain Name System (DNS)
How DNS stores RSA public keys and RSA/SHA-1 signatures, replacing the weaker MD5-based scheme.
Eastlake 3rd, D., "RSA/SHA-1 SIGs and RSA KEYs in the Domain Name System (DNS)", RFC 3110, DOI 10.17487/RFC3110, May 2001, <https://www.rfc-editor.org/info/rfc3110>.
@misc{rfc3110,
series = {Request for Comments},
number = 3110,
howpublished = {RFC 3110},
publisher = {RFC Editor},
doi = {10.17487/RFC3110},
url = {https://www.rfc-editor.org/info/rfc3110},
author = {D. Eastlake 3rd},
title = {{RSA/SHA-1 SIGs and RSA KEYs in the Domain Name System (DNS)}},
pagetotal = 7,
year = 2001,
month = may,
} RFC text
Contents
Network Working Group D. Eastlake 3rd
Request for Comments: 3110 Motorola
Obsoletes: 2537 May 2001
Category: Standards Track
RSA/SHA-1 SIGs and RSA KEYs in the Domain Name System (DNS)
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract
This document describes how to produce RSA/SHA1 SIG resource records
(RRs) in Section 3 and, so as to completely replace RFC 2537,
describes how to produce RSA KEY RRs in Section 2.
Since the adoption of a Proposed Standard for RSA signatures in the
DNS (Domain Name Space), advances in hashing have been made. A new
DNS signature algorithm is defined to make these advances available
in SIG RRs. The use of the previously specified weaker mechanism is
deprecated. The algorithm number of the RSA KEY RR is changed to
correspond to this new SIG algorithm. No other changes are made to
DNS security.
Acknowledgements
Material and comments from the following have been incorporated and
are gratefully acknowledged:
Olafur Gudmundsson
The IESG
Charlie Kaufman
Steve Wang
Table of Contents
1. Introduction................................................... 2
2. RSA Public KEY Resource Records................................ 3
3. RSA/SHA1 SIG Resource Records.................................. 3
4. Performance Considerations..................................... 4
5. IANA Considerations............................................ 5
6. Security Considerations........................................ 5
References........................................................ 5
Author's Address.................................................. 6
Full Copyright Statement.......................................... 7
1. Introduction
The Domain Name System (DNS) is the global hierarchical replicated
distributed database system for Internet addressing, mail proxy, and
other information [RFC1034, 1035, etc.]. The DNS has been extended
to include digital signatures and cryptographic keys as described in
[RFC2535]. Thus the DNS can now be secured and used for secure key
distribution.
Familiarity with the RSA and SHA-1 algorithms is assumed [Schneier,
FIP180] in this document.
RFC 2537 described how to store RSA keys and RSA/MD5 based signatures
in the DNS. However, since the adoption of RFC 2537, continued
cryptographic research has revealed hints of weakness in the MD5
[RFC1321] algorithm used in RFC 2537. The SHA1 Secure Hash Algorithm
[FIP180], which produces a larger hash, has been developed. By now
there has been sufficient experience with SHA1 that it is generally
acknowledged to be stronger than MD5. While this stronger hash is
probably not needed today in most secure DNS zones, critical zones
such a root, most top level domains, and some second and third level
domains, are sufficiently valuable targets that it would be negligent
not to provide what are generally agreed to be stronger mechanisms.
Furthermore, future advances in cryptanalysis and/or computer speeds
may require a stronger hash everywhere. In addition, the additional
computation required by SHA1 above that required by MD5 is
insignificant compared with the computational effort required by the
RSA modular exponentiation.
This document describes how to produce RSA/SHA1 SIG RRs in Section 3
and, so as to completely replace RFC 2537, describes how to produce
RSA KEY RRs in Section 2.
Implementation of the RSA algorithm in DNS with SHA1 is MANDATORY for
DNSSEC. The generation of RSA/MD5 SIG RRs as described in RFC 2537
is NOT RECOMMENDED.
The key words "MUST", "REQUIRED", "SHOULD", "RECOMMENDED", "NOT
RECOMMENDED", and "MAY" in this document are to be interpreted as
described in RFC 2119.
2. RSA Public KEY Resource Records
RSA public keys are stored in the DNS as KEY RRs using algorithm
number 5 [RFC2535]. The structure of the algorithm specific portion
of the RDATA part of such RRs is as shown below.
Field Size
----- ----
exponent length 1 or 3 octets (see text)
exponent as specified by length field
modulus remaining space
For interoperability, the exponent and modulus are each limited to
4096 bits in length. The public key exponent is a variable length
unsigned integer. Its length in octets is represented as one octet
if it is in the range of 1 to 255 and by a zero octet followed by a
two octet unsigned length if it is longer than 255 bytes. The public
key modulus field is a multiprecision unsigned integer. The length
of the modulus can be determined from the RDLENGTH and the preceding
RDATA fields including the exponent. Leading zero octets are
prohibited in the exponent and modulus.
Note: KEY RRs for use with RSA/SHA1 DNS signatures MUST use this
algorithm number (rather than the algorithm number specified in the
obsoleted RFC 2537).
Note: This changes the algorithm number for RSA KEY RRs to be the
same as the new algorithm number for RSA/SHA1 SIGs.
3. RSA/SHA1 SIG Resource Records
RSA/SHA1 signatures are stored in the DNS using SIG resource records
(RRs) with algorithm number 5.
The signature portion of the SIG RR RDATA area, when using the
RSA/SHA1 algorithm, is calculated as shown below. The data signed is
determined as specified in RFC 2535. See RFC 2535 for fields in the
SIG RR RDATA which precede the signature itself.
hash = SHA1 ( data )
signature = ( 01 | FF* | 00 | prefix | hash ) ** e (mod n)
where SHA1 is the message digest algorithm documented in [FIP180],
"|" is concatenation, "e" is the private key exponent of the signer,
and "n" is the modulus of the signer's public key. 01, FF, and 00
are fixed octets of the corresponding hexadecimal value. "prefix" is
the ASN.1 BER SHA1 algorithm designator prefix required in PKCS1
[RFC2437], that is,
hex 30 21 30 09 06 05 2B 0E 03 02 1A 05 00 04 14
This prefix is included to make it easier to use standard
cryptographic libraries. The FF octet MUST be repeated the maximum
number of times such that the value of the quantity being
exponentiated is one octet shorter than the value of n.
(The above specifications are identical to the corresponding parts of
Public Key Cryptographic Standard #1 [RFC2437].)
The size of "n", including most and least significant bits (which
will be 1) MUST be not less than 512 bits and not more than 4096
bits. "n" and "e" SHOULD be chosen such that the public exponent is
small. These are protocol limits. For a discussion of key size see
RFC 2541.
Leading zero bytes are permitted in the RSA/SHA1 algorithm signature.
4. Performance Considerations
General signature generation speeds are roughly the same for RSA and
DSA [RFC2536]. With sufficient pre-computation, signature generation
with DSA is faster than RSA. Key generation is also faster for DSA.
However, signature verification is an order of magnitude slower with
DSA when the RSA public exponent is chosen to be small as is
recommended for KEY RRs used in domain name system (DNS) data
authentication.
A public exponent of 3 minimizes the effort needed to verify a
signature. Use of 3 as the public exponent is weak for
confidentiality uses since, if the same data can be collected
encrypted under three different keys with an exponent of 3 then,
using the Chinese Remainder Theorem [NETSEC], the original plain text
can be easily recovered. If a key is known to be used only for
authentication, as is the case with DNSSEC, then an exponent of 3 is
acceptable. However other applications in the future may wish to
leverage DNS distributed keys for applications that do require
confidentiality. For keys which might have such other uses, a more
conservative choice would be 65537 (F4, the fourth fermat number).
Current DNS implementations are optimized for small transfers,
typically less than 512 bytes including DNS overhead. Larger
transfers will perform correctly and extensions have been
standardized [RFC2671] to make larger transfers more efficient, it is
still advisable at this time to make reasonable efforts to minimize
the size of KEY RR sets stored within the DNS consistent with
adequate security. Keep in mind that in a secure zone, at least one
authenticating SIG RR will also be returned.
5. IANA Considerations
The DNSSEC algorithm number 5 is allocated for RSA/SHA1 SIG RRs and
RSA KEY RRs.
6. Security Considerations
Many of the general security considerations in RFC 2535 apply. Keys
retrieved from the DNS should not be trusted unless (1) they have
been securely obtained from a secure resolver or independently
verified by the user and (2) this secure resolver and secure
obtainment or independent verification conform to security policies
acceptable to the user. As with all cryptographic algorithms,
evaluating the necessary strength of the key is essential and
dependent on local policy. For particularly critical applications,
implementers are encouraged to consider the range of available
algorithms and key sizes. See also RFC 2541, "DNS Security
Operational Considerations".
References
[FIP180] U.S. Department of Commerce, "Secure Hash Standard", FIPS
PUB 180-1, 17 Apr 1995.
[NETSEC] Network Security: PRIVATE Communications in a PUBLIC
World, Charlie Kaufman, Radia Perlman, & Mike Speciner,
Prentice Hall Series in Computer Networking and
Distributed Communications, 1995.
[RFC1034] Mockapetris, P., "Domain Names - Concepts and Facilities",
STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain Names - Implementation and
Specification", STD 13, RFC 1035, November 1987.
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2437] Kaliski, B. and J. Staddon, "PKCS #1: RSA Cryptography
Specifications Version 2.0", RFC 2437, October 1998.
[RFC2535] Eastlake, D., "Domain Name System Security Extensions",
RFC 2535, March 1999.
[RFC2536] Eastlake, D., "DSA KEYs and SIGs in the Domain Name System
(DNS)", RFC 2536, March 1999.
[RFC2537] Eastlake, D., "RSA/MD5 KEYs and SIGs in the Domain Name
System (DNS)", RFC 2537, March 1999.
[RFC2541] Eastlake, D., "DNS Security Operational Considerations",
RFC 2541, March 1999.
[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC
2671, August 1999.
[Schneier] Bruce Schneier, "Applied Cryptography Second Edition:
protocols, algorithms, and source code in C", 1996, John
Wiley and Sons, ISBN 0-471-11709-9.
Author's Address
Donald E. Eastlake 3rd
Motorola
155 Beaver Street
Milford, MA 01757 USA
Phone: +1-508-261-5434 (w)
+1-508-634-2066 (h)
Fax +1-508-261-4777 (w)
EMail: Donald.Eastlake@motorola.com
Full Copyright Statement
Copyright (C) The Internet Society (2001). All Rights Reserved.
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
AI ELI5
On this page
§1Why MD5 Was Replaced
The Domain Name System (DNS) had been extended with digital signatures and cryptographic keys, so DNS data could be secured and used to distribute keys. RFC 2537 described how to store RSA keys and RSA/MD5 signatures in the DNS.
Since then, cryptographic research had revealed hints of weakness in the MD5 hash. SHA-1 produces a larger hash and was by then generally acknowledged to be stronger. The RFC says a stronger hash is probably not needed today in most secure zones, but that critical zones such as the root, most top level domains and some second and third level domains are valuable enough targets that it would be negligent not to offer it.
It also notes that the extra computation for SHA-1 compared with MD5 is insignificant next to the RSA modular exponentiation.
The old way of making a fingerprint of a message was starting to look a bit weak. A stronger fingerprint costs almost nothing extra, so it is worth switching.
§2What the RFC Changes and Requires
The document describes how to produce RSA/SHA1 SIG resource records and, so that it completely replaces RFC 2537, also how to produce RSA KEY records. The algorithm number of the RSA KEY record is changed to match the new signature algorithm. No other changes are made to DNS security.
Two requirements are stated up front. Implementing RSA with SHA1 in DNS is MANDATORY for DNSSEC, and generating RSA/MD5 SIG records as described in RFC 2537 is NOT RECOMMENDED. The use of the earlier, weaker mechanism is deprecated.
The rule is simple: use the new stronger stamp, and please stop using the old one.
§3RSA Public KEY Records
RSA public keys are stored in the DNS as KEY records with algorithm number 5. The algorithm-specific part of the record data holds three things in order: the exponent length (1 or 3 octets), the exponent itself, and the modulus, which takes the remaining space.
The exponent length is one octet if the exponent is 1 to 255 octets long. If it is longer than 255 octets, it is a zero octet followed by a two-octet unsigned length. The modulus length is not stored; it is worked out from RDLENGTH and the fields before it. Leading zero octets are prohibited in both the exponent and the modulus.
For interoperability, the exponent and modulus are each limited to 4096 bits. KEY records used with RSA/SHA1 signatures MUST use algorithm number 5 rather than the number from the obsoleted RFC 2537.
A public key is stored as a short note: first how long one number is, then that number, then the rest, which is a second number. Keeping the layout fixed means everybody reads it the same way.
| Field | Size |
|---|---|
| exponent length | 1 or 3 octets |
| exponent | as specified by length field |
| modulus | remaining space |
§4RSA/SHA1 SIG Records
RSA/SHA1 signatures are stored in SIG records, also with algorithm number 5. The data to be signed is determined as in RFC 2535. The signer first computes hash = SHA1 ( data ) and then the signature as ( 01 | FF* | 00 | prefix | hash ) ** e (mod n).
Here | is concatenation, e is the signer's private key exponent and n is the modulus of the signer's public key. The octets 01, FF and 00 are fixed. The prefix is the ASN.1 BER SHA1 algorithm designator prefix from PKCS1, which is hex 30 21 30 09 06 05 2B 0E 03 02 1A 05 00 04 14. It is included to make it easier to use standard cryptographic libraries. These specifications are identical to the corresponding parts of PKCS #1.
The FF octet MUST be repeated the maximum number of times such that the value being exponentiated is one octet shorter than n. The size of n MUST be not less than 512 bits and not more than 4096 bits, and n and e SHOULD be chosen so the public exponent is small. Leading zero bytes are permitted in the signature.
The signer shrinks the message to a fingerprint, pads it out in a fixed way, and locks it with a secret number. Anyone with the matching public number can check the lock.
hash = SHA1 ( data )
signature = ( 01 | FF* | 00 | prefix | hash ) ** e (mod n)
prefix = hex 30 21 30 09 06 05 2B 0E 03 02 1A 05 00 04 14§5Performance Considerations
Signature generation speeds are roughly the same for RSA and DSA, and with enough pre-computation DSA is faster. Key generation is also faster for DSA. But verifying a signature is an order of magnitude slower with DSA when the RSA public exponent is small, which is the recommended choice for KEY records used to authenticate DNS data.
A public exponent of 3 minimizes the effort to verify a signature. It is weak for confidentiality, because the same data encrypted under three different keys with exponent 3 can be recovered using the Chinese Remainder Theorem. That is acceptable for keys used only for authentication, as in DNSSEC. For keys that might later serve other uses, a more conservative choice is 65537.
The RFC also notes that DNS implementations are optimized for transfers typically under 512 bytes, so it is advisable to keep KEY record sets small consistent with adequate security. A secure zone also returns at least one SIG record alongside them.
A small secret-lock number makes checking fast. That is fine when you only need to prove who sent something, but not when you need to keep a message private.
§6IANA and Security Considerations
For IANA, DNSSEC algorithm number 5 is allocated for both RSA/SHA1 SIG records and RSA KEY records.
Many of the security considerations in RFC 2535 apply. Keys retrieved from the DNS should not be trusted unless they were securely obtained from a secure resolver or independently verified by the user, and that process meets the user's security policies. How strong a key needs to be depends on local policy, and for critical applications implementers should consider the range of algorithms and key sizes. The RFC points to RFC 2541 for operational advice.
Do not trust a key just because it showed up. Make sure you got it from a safe source, and pick a strong enough lock for important things.